Hydrocracking is an important process for producing middle distillate fuels from heavier feedstocks, such as vacuum gas oils. In the hydrocracking process, heavy feeds, which generally contain relatively large amounts of sulfur and nitrogen, are cracked into lighter, lower boiling hydrocrackate products for use as fuels, petrochemical feedstocks, and other petroleum refinery products. Hydrocracking catalysts are generally selected for high cracking activity, and for resisting the poisoning effects of the sulfur and nitrogen-containing materials in the feedstock. To this end, Y-type zeolites are often included as components of the hydrocracking catalyst. These zeolites catalyze cracking reactions at much higher rates than amorphous (i.e. non-zeolitic) catalysts. In addition, Y-type zeolites can be tailored to provide a range of cracking activity, depending, for example, on the relative amounts of silica and alumina in the crystalline matrix of the zeolite. Zeolites with a low SiO.sub.2 /Al.sub.2 O.sub.3 ratio exhibit high cracking activity. As the alumina is removed from the crystalline matrix by methods known to the art, the SiO.sub.2 /Al.sub.2 O.sub.3 increases, and the relative cracking activity decreases. The unit cell size of the crystalline zeolite also tends to decrease with increasing SiO.sub.2 /Al.sub.2 O.sub.3 ratios. Zeolites with a relatively large unit cell size have high cracking activity. By reducing the unit cell size using methods known in the art, the cracking activity decreases. However, it is possible to exploit the decrease in activity to tailor a catalyst which provides a higher selectivity of a desired cracked product. Methods have also been developed to produce zeolites having increasingly smaller crystallite sizes, and the published literature teach the use of these small crystallite size zeolites for hydrocarbon conversion processes.
U.S. Pat. No. 5,401,704 discloses a hydrocracking process using a catalyst comprising zeolite Y and a combination of hydrogenating metals, where the zeolite Y has a crystal size of from about 0.1 to about 0.5 microns. According to U.S. Pat. No. 5,401,704, the small crystal Y zeolite provides high selectivity for producing C.sub.5 -165.degree. C. naphtha.
A number of patents disclose hydrocracking and/or hydrotreating processes using zeolite containing catalysts, the zeolite component having relatively high SiO.sub.2 /Al.sub.2 O.sub.3 ratios or relatively small unit cell sizes. For example, Kirker, in U.S. Pat. No. 5,171,422 teaches hydrocracking a feedstock with a catalyst comprising a zeolite of the faujasite structure possessing a framework silica: alumina ratio of at least about 50:1.
Partridge, et al., in U.S. Pat. No. 4,820,402, teaches a hydrocracking process using a catalyst comprising a hydrogenation component and a zeolite which has pores with a dimension greater than 6 Angstroms and a hydrocarbon sorption capacity for hexane of at least 6 percent and has a framework silica:alumina ratio of at least about 50:1. Partridge, et al., further suggest that the selectivity for production of the higher boiling distilled range product is preferentially increased in the hydrocracking process.
Absil, et al., in U.S. Pat. Nos. 5,401,704 and 5,620,590 teaches a catalyst comprising a zeolite Y with a crystal size of from about 0.1 to about 0.5 microns and a unit cell size of 24.5 Angstroms or less for hydrocracking a variety of feedstock.
U.S. Pat. No. 5,565,088, issued to Nair, et al., discloses a process for upgrading middle distillates by hydrocracking a feedstream boiling above 350.degree. C. with a hydrocracking catalyst comprising a Y zeolite, and contacting the product stream with a dewaxing catalyst comprising an intermediate pore non-zeolitic molecular sieve material and from about 0.1 to about 0.75 wt % of a sulfided non-noble metal hydrogenation component. A Y-type zeolite preferred by Nair, et al., possesses a unit cell size between about 24.20 Angstroms and 24.45 Angstroms.
Others describe layered catalyst systems. For example, Winslow, et al., in U.S. Pat. No. 4,990,243 teaches a denitrification process using a layered catalyst system comprising a first layer of a catalyst which comprises a nickel-molybdenum-phosphorous/alumina catalyst or a cobalt-molybdenum-phosphorous/alumina catalyst and comprising a second layer of a catalyst which comprises a nickel-tungsten/silica-alumina-zeolite or a nickel-molybdenum/silica-alumina-zeolite catalyst. Habib et al., in U.S. Pat. Nos. 5,439,860 and 5,593,570 teach a dual function catalyst system for combined hydrotreating and hydrocracking process operations using randomly intermixed hydrodenitrification and/or hydrodesulfurization catalyst and hydrocracking catalyst. The preferred hydrocracking catalyst of Habib, et al. comprises a Y zeolite having a unit cell size greater than about 24.55 Angstroms and a crystal size less than about 2.8 microns. Habib, et al., in U.S. Pat. No. 5,393,410 teaches a conversion process using catalyst comprising an ultra stable Y zeolite base, wherein the Y zeolite has a unit cell size greater than about 24.55 Angstroms and a crystal size less than about 2.8 microns.
While the catalysts described in the patents listed above have high cracking activity, the need remains for a catalyst system which can maintain adequate catalyst life while producing higher amounts of the desired hydrocracked product.
During hydrocracking within a catalytic reaction zone a petroleum feedstock is introduced into the zone at a first reaction temperature. As the reacting oil passes through the zone, exothermic hydrocracking reactions increase the temperature of the oil and of the catalyst which the oil contacts, so that the temperature in the zone increases through the zone in the direction of flow of oil. Thus, a steep temperature profile through the zone indicates a rapid temperature increase through the zone. Methods of adjusting the temperature, e.g. adding cool quench hydrogen or quench oil at intermediate locations in the zone, are known and commonly practiced. However, the amount of heat generated by reaction is such that a second reaction temperature, which is the temperature of the oil exiting the zone, is generally higher than the first reaction temperature. In a hydrocracking reactor which contains a layered catalyst system, there is a generally increasing temperature profile through the entire system, such that a second reactor temperature, which is the temperature of the oil exiting the system, is higher than the first reaction temperature. While hydrocracking at a high exit temperature, and with a steep temperature profile along the hydrocracking reactor, often causes significant reduction in reaction selectivity, results in poor product quality, and leads to reduced catalyst life, the refiner is often constrained to run the hydrocracker at such conditions for processing, economic, or other reasons. A more selective catalyst system for operating under a steep temperature profile is desired.